The best tables for this
purpose are given in Lunge and Hurter's _Alkali-Makers' Pocket-Book_,
but for ordinary purposes of calculation in the works or factory, a
convenient relationship exists in the case of hydrochloric acid between
specific gravity and percentage of real acid, such that specific gravity
as indicated by Twaddell's hydrometer directly represents percentage of
real acid in any sample of hydrochloric acid.
The point at which neutralisation of an acid by alkali or _vice versa_
just takes place is ascertained very accurately by the use of certain
sensitive colours. At first litmus and cochineal tinctures were used,
but in testing crude alkalis containing alumina and iron, it was found
that lakes were formed with these colours, and they become precipitated
in the solution, and so no longer sensitive. The chemist was then
obliged to resort to certain sensitive coal-tar colours, which did not,
as the dyer and printer knew, form lakes with alumina and iron, such as
methyl orange, fluorescein, Congo red, phenolphthalein, and so forth.
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